1. Technical Field
This invention relates to systems and techniques for providing range measurements, and more particularly to a field shift moire system for providing such measurements.
2. Discussion
Difficulties in processing visual information in three spatial dimensions are encountered in a number of areas and particularly in the field of machine vision. A number of approaches for providing three dimensional machine vision includes structured lighting, range finding, shape from shading and stereo viewing. Structured lighting techniques, while popular because of their simplicity, are time consuming if full three dimensional data is needed, since the surface typically must be scanned to build up the full shape information. Laser range finding systems produce a full three dimensional map but typically must scan over the surface one point at a time to do so. Shape from shading techniques provide the shape information in one video frame by observing the lighting variations which occur due to the varying angle of incidence that directional lighting has on varying surface slopes. Great care must be taken in such techniques to account for variations in the illumination light and variations in surface reflectivity, both of which can produce erroneous data. Stereo viewing techniques triangulate off key points on the part surface, as seen from two different perspectives. However, correlating such points is a difficult task, and if the surface is smooth and featureless, there is nothing to correlate.
While all of the above have their applications, they do have a number of limitations. In particular large computing capacity is often required as well as the requirement of highly skilled operators. Further, these techniques may not be suitable when precision full-field part measurement is required. Without full-field object data some imperfections can be missed altogether in the surface to be analyzed. While coordinate measuring machines can be programmed to measure specific parts, these systems are generally slow and therefore measurements for quality control, for example, are often limited to a spot check system. Thus it would be desirable to have a versatile automated contouring system capable of measuring either large or even small areas at a time to provide the opportunity for better and more complete inspections.
Interferometric techniques have long been used to obtain high resolution, full-field shape information. For example, holographic interferometry provides a very high sensitivity to changes in a structure due to stress, heat, vibrations, flaws, or deformations. However, one of the primary strengths of holographic interferometry, high sensitivity, is also a primary limitation. This is because the amplitudes of the changes being measured are often beyond the upper range of holographic interferometry.
Another full-field non-contact measurement technique, moire interferometry offers many of the testing capabilities of holographic interferometry with an important difference. The sensitivity in moire interferometry can be adjusted to fit the application requirements. As a result, moire interferometry can be very tolerant to positioning errors or extraneous motions. A moire pattern is made by forming a subject grating, by projecting, shadowing, or contacting a grating onto the object to be measured, and comparing this grating to some reference grating by overlaying the two grating images. If the reference grating is a straight line grating, the beat pattern between the two gratings will form a contour map of the object's surface in the same way that a topographical map delineates the contours of land.
One problem that has persisted in moire interferometry over the years has been the lack of ability to obtain an absolute measurement from interferograms with digital heterodyne techniques, such as phase shifting techniques. See Albert J. Boehnlein, Kevin G. Harding "Adaptation of a parallel architecture computer to phase shifted moire interferometry" SPIE volume 728, Optics, illumination And Image Sensing For Machine Vision, page 183 (1986), which is hereby incorporated by reference. This problem is due to the fact that a static interferogram suffers from the lack of information to distinguish a hill from a valley. By shifting the phase of the fringe pattern, the sign of a slope can be determined, but there remains an ambiguity when the surface in question has a discontinuous jump. To determine the shape of such a discontinuous or prismatic surface with block structures as part of the shape, the measurement needs to be absolute, not just relative to connecting points.
The reason that one cannot get absolute numbers from the phase shift equations is that the equations rely on the arc-tangent function, which is only continuous over -.pi./ 2 to .pi./2. With a two input arc-tangent function, one can determine the quadrant, and therefor the phase over the interval -.pi. to .pi.. The inability to determine the absolute phase is termed the modulo two .pi. or two .pi. ambiguity problem. With phase shifting it is possible to make relative measurements of the points on the interferogram, provided that the surface has no discontinuities greater than the contour interval.
Thus, it would be desirable to provide a technique for providing absolute range measurements that is a non contact full-field measurement technique. It is further desirable to provide such a technique which does not have the two .pi. ambiguity problem. Further, it is desirable to provide a system which is applicable to industrial environments, and which can be applied to fast parallel process computers for fast absolute contour generation of prismatic parts.